The present invention relates generally to pigments composed of coated TiO.sub.2 particles, and particularly to TiO.sub.2 pigments having coatings comprising cerium cations, selected acid anions, and an outer coating of alumina.
TiO.sub.2 has a high refractive index for its density, which renders it a superior pigment for use in coatings, e.g., paints. However, TiO.sub.2 is photoactive; exposure to ultraviolet radiation results in generation of free radicals on the surface of TiO.sub.2 particles. Thus, where a TiO.sub.2 pigment is employed in a paint exposed to sunlight, free radicals are generated which can migrate to the film-forming component of the paint, resulting in coating degradation or failure.
Therefore, minimizing free radical migration is desirable in order to provide lightfast and stable pigments for coating use. Heretofore, the majority of commercial TiO.sub.2 pigments have been coated with dense silica or densified alumina to provide lightfastness. However, some patent references have disclosed use of cerium in association with TiO.sub.2 pigments to promote lightfastness.
Lederer, U.S. Pat. No. 3,513,007, discloses TiO.sub.2 particles having coatings formed from dispersions containing water-soluble hydrolyzable compounds of silicon, titanium, zirconium, or phosphorus, to which water-soluble hydrolyzable compounds of aluminum, cerium, and or calcium and alkali are added.
Barnard, U.S. Pat. No. 4,239,548, discloses pigments having inner coatings comprising cerium and phosphate radicals and outer coatings comprising aluminum and phosphate radicals and optionally hydrous alumina.
Jacobson, U.S. Pat. No. 4,460,655, describes pigments of TiO.sub.2 particles associated with compounds selected from the group consisting of Na.sub.7 Ce.sub.6 F.sub.31, K.sub.7 Ce.sub.6 F.sub.31, Li.sub.7 Ce.sub.6 F.sub.31, and (NH.sub.4).sub.7 Ce.sub.6 F.sub.31.
Jacobson, U.S. Pat. No. 4,461,810, discloses pigments of TiO.sub.2 particles, the surfaces of which are associated with cerium cations and sulfate, phosphate, or silicate anions. The particles are also provided with an outer coating of alumina.
Newland et al., U.S. Pat. No. 4,022,632, describes TiO.sub.2 pigments treated with carboxylate salts of cobalt, cerium, or manganese.
Bramekamp, U.S. Pat. No. 3,506,466, discloses TiO.sub.2 pigment particles coated with inorganic oxides, where the inorganic oxide bears on its surface an amine salt of an oxycarboxylic acid selected from the group consisting of lactic, citric, malic, tartaric, and glycolic acids.
It has now been found that pigments of TiO.sub.2 particles treated with cerium cations in association with certain polyfunctional water-soluble acid anions or borate ions, and then topcoated with alumina, exhibit excellent lightfastness, gloss, and dispersion characteristics.